JPS5885580A - Manufacture of tunnel type josephson junction - Google Patents

Abstract

PURPOSE:To remarkably reduce a leakage current in a sub cap region in a tunnel type Josephson junction U by heat treating a superconductive layer in atmosphere containing no oxygen. CONSTITUTION:A silicon substrate is used as an insulating substrate 1, a superconductive layer 2 is formed of Pb-In-Au alloy in the first step, and the surface 3 purified from the layer 2 is obtained by high frequency sputtering with argon gas in the second step. A high frequency discharge oxidation is performed in the third step to form an oxidized film 4 as an insulating film 5 to become tunnel barrier on the surface, and the second superconductive layer 6 is formed in the fourth step. In this invention, a tube is evacuated in high vacuum after the second step before the third step, and the layer 2 is heat treated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized in that a first superconductor layer having a desired pattern is formed on a substrate, and an insulating film serving as a tunnel barrier is formed on the surface of a PfT layer of the first superconductor layer. The present invention relates to an improvement in the manufacturing method of a tunnel-type Josephson junction having a structure in which a second superconductor layer extends through a membrane.

Conventionally, as shown in FIG. 1, the method for manufacturing a tunnel-type Josephun junction includes a first step (first step) of forming a first superconductor having a desired pattern on a substrate 1 obtained in advance. 1-A) and, after the first step, a second step of cleaning the required surface of the first superconductor layer 2 to a surface that has been cleaned by spuck-linking treatment using an inert gas. (FIG. 1B), and after the second step, on the cleaned surface 6 of the first superconductor layer 2 (this step, the oxidized film 4 is formed as a tunnel barrier by a frequency discharge oxidation process). The sixth step of forming the film 5 (FIG. 1C) and the termination of the sixth step 5
A fourth layer for forming a second superconductor layer 6 on the membrane 5.
A method has been proposed to obtain the desired tunnel-type Josephson junction U by including the steps (FIG. 1D).

Then, a second step (6) of turning the surface of the first superconductor (formed on the insulating plate 1) vIi2 to the cleaned surface 3 is carried out. 1B), even if the first superconductor layer 2 is obtained with a natural oxide film formed on the required surface, the first superconductor layer 2 is formed with the natural oxide film removed. Since the oxide film 4 is formed on the required surface of the superconductor layer 2, the intended tunnel-type Josephson junction U is formed using the insulating film 5 that becomes the tunnel barrier.
can be obtained by forming only the oxide film 4. On the other hand, since the oxide film 4 as the insulating film 5 which becomes the tunnel barrier is formed by high frequency TM processing, other vapor phase growth processing and thermal oxidation are not required. It can be easily formed as an excellent insulating film in a short time with better control than when it is formed by processing, etc. Therefore, the target tunnel type Josephson junction U can be made to have the desired excellent characteristics. It has the characteristic that it can be easily obtained.

However, in the case of the conventional method described above in FIG. 1, the tunnel type Josephson junction U obtained by that method is
As seen in the voltage (V)-current (I) characteristic shown by the solid line in FIG. 2, the resistance value R6 in the sub-subcap region A is larger than the resistance value RN in other regions. However, the resistance value R6 and the ratio R()/'~ can only be obtained as relatively small, and therefore, a relatively large leakage current is involved in the subgap region A. It had the disadvantage that it could only be obtained as a product.

In view of the above-mentioned problem, the inventors of the present invention, as a result of various experiments, adopted the sequential steps 1 to 4 described above in FIG.
After the step and before the sixth step, the first superconductor layer 2
If heat treatment is performed in an oxygen-containing atmosphere, the resistance ratio of the target tunnel-type di′:′:sefunn junction U as described above, △, will be significantly greater than in the case without such heat treatment. Therefore, we have confirmed that the leakage current in the sub-gap region mentioned above can be obtained as being much smaller than in the case where such heat treatment is not performed. Ta.

Furthermore, although the first to fourth steps described above in FIG. Not in an atmosphere containing
If a thermal oxide film is thereby formed on the surface of the first superconductor layer 2, the target tunnel-type Josephson junction U can be formed such that the above-mentioned resistance ratio R, /π does not undergo such heat treatment. Therefore, the leakage current in the sub-gap region described above can be obtained as being much smaller than when the proper heat treatment is not performed. I also came to confirm this.

Heat treatment is performed on the first superconductor layer 2 in an oxygen-containing atmosphere, and then in an oxygen-containing atmosphere. By forming the oxide film, it is possible to obtain the desired tunnel-type Josephson junction U with the above-mentioned resistance ratio RG/1 moat being much larger than in the case where such heat treatment is not performed, For this reason, it has been confirmed that the leakage current in the above-mentioned sub-gap region can be significantly reduced compared to the case (b) in which no heat treatment is performed.

Therefore, the present inventors hereby claim that the first and second claims
This has led us to propose the present invention as described in Section 6, which will become clearer from the description of the embodiments of the present invention below.

In the embodiment of the present invention, although the first to fourth steps described above in FIG. 1 are carried out, a silicon substrate is used as the insulating substrate 1, and the first step (see In A), the first superconductor layer 2 is made of Pb-1,n-Au alloy (In
: '12 vehicle weight%, Au: 4% by weight). In the second step (FIG. 1B), the sputtering process using inert dregs is performed as a high-frequency sputter link process using argon gas at a power of 150 W and a time of 6 minutes to form the first superconductor N2. A cleaned surface 3 was obtained. Further, in the sixth step (FIG. 1 0), the cleaned surface 6 of the first superconductor layer 2 is subjected to a cylindrical frequency discharge oxidation treatment as a high frequency discharge oxidation treatment with a power of 25 W for 1 hour and 10 minutes. An oxide film 4 as an insulating film 5 which becomes a tunnel barrier.
was formed. In the fourth step (FIG. 1D), the second superconductor layer 6 was formed of a pb-Bi gold alloy (Bi: 29% by weight).

According to the present invention, after the second step and before the third step, the following heat treatment was performed assuming that the second and sixth steps were performed in the same tube.

(1) The argon gas introduced into the tube during the high-frequency sputtering process using argon gas in the second step is evacuated to create a high vacuum state of 2×10 Torr inside the tube, and in that state, the first superconductor Body layer 2& was subjected to heat treatment at about 70° C. for about 6 hours.

In such a case, the desired tunnel type Josephson junction U
However, the above-mentioned resistance ratio R7~ was obtained as having a large value of about 14.5. Incidentally, if such heat treatment is not performed, the desired tunnel type Josephson junction U
However, with the resistance ratio R7~ mentioned above, only a small value of about 11.2 could be obtained. Therefore, the target tunnel-type Josephson junction U has a leakage current in the sub-gap region which is significantly smaller than that without such heat treatment.

(2) The argon gas that was introduced into the tube during the high-frequency sputtering process using argon gas in the second step is exhausted, the inside of the tube is brought to a high vacuum state of 2 x 10-7 TOrr, and then the oxygen gas is introduced into the tube. Then,'a
An atmosphere containing oxygen that suppresses the pressure of 7×10' Torr is created inside the interior, and in that state, the first superconductor layer 2 is heat-treated at about 70° C. for about 3 hours.
A thin thermal oxide film was formed on the desired fk# coating Tki 3.

In such a case, the desired tunnel type Josephson junction U
However, the above-mentioned resistance ratio R^ was expected to have a large value of about 15.5. Therefore, the leakage current in the target tunnel type Josephson junction U or the sub-gap region was significantly smaller than that in the case without such heat treatment.

(31 Since argon gas was used in the second step, the argon gas that had been introduced into the tube during the frequency sputtering process was evacuated to create a high vacuum state of 2 x 10 Torr in the tube, and in that state, the first super The conductor layer 2 is heat-treated at about 70° C. for about 1 hour, and then oxygen gas is introduced into the tube to create an oxygen-containing atmosphere with a pressure of 7×10 −2 Torr. The superconductor layer 2 was subjected to heat treatment at about 70° C. for about 2 hours to form a thin thermal oxide film on the required cleaned surface 3 of the first superconductor layer 2.

In such a case, the desired tunnel type Josephson junction U
is the resistance ratio R,,/H mentioned above.

It was obtained as having a dog value of about 15.8.

Therefore, the desired tunnel-type Josephson junction U was obtained with significantly smaller leakage current in the sub-gap region than in the case without such heat treatment.

In the above description, the first superconductor layer 2 has a composition of 12% by weight of I+ and 4mf-1i% of Au.
Although the embodiment has been described in which the first superconductor layer 2 is made of an In-Au alloy, the first superconductor layer 2 has a composition other than the one just described.
Even in the case of the 'b-I n-Au alloy, the excellent effects described above were obtained. Furthermore, although an example has been described in which the sputtering process using an inert gas in the second step is a high frequency sputtering process using an argon gas, the sputtering process using an inert gas is a high frequency sputtering process using an argon gas, Even in the case of ion beam sputtering treatment using krypton gas, the excellent effects described above were obtained. Further, an example has been described in which the heat treatment after the second step and before the sixth step is a heat treatment at a temperature of about 70°C, but even if the heat treatment is a heat treatment at a temperature of 70°C ± 10°C, The excellent effects described above were obtained.

Also, the above (1) and (2) after the second step and before the sixth step
The above-mentioned examples have been described in which the heat treatment is for 6 hours, but the above-mentioned advantages can be achieved even if the heat treatment is for a time slightly shorter than 6 hours or for a time slightly longer than 3 hours. The effect was obtained. Furthermore, an example has been described in which the heat treatment in (3) above after the second step and before the third step is a heat treatment for 1 hour followed by a heat treatment for 2 hours, but the heat treatment may be slightly shorter or longer than 1 hour. The above-mentioned excellent effects were obtained even when the heat treatment was performed for a period of time followed by a heat treatment for a period somewhat shorter or longer than 2 hours.

In the above, only a few embodiments of the present invention have been described; however, the first and second superconductor layers may be made of a lead alloy, but other superconductors may be used. It will be obvious that even if the present invention is applied, the above-mentioned effects can be obtained. Also second
It will be clear that even if the heat treatment after the step 2 and before the 6th step is performed at a different time and temperature than in the above-mentioned embodiments, the above-mentioned excellent effects can be obtained.

[Brief explanation of the drawing]

Figures 1A to 1D are cross-sectional views showing sequential steps in an embodiment of the method for manufacturing a tunnel-type Josephson junction according to the present invention, and Figure 2 is a diagram showing voltage-current characteristics of the tunnel-type Josephson junction. It is. In the figure, 1 is a substrate, 2 is a first superconductor layer, 6 is a cleaned surface, 4 is a fermented film, 5 is an insulating film, and 6 is a W2 superconductor layer. Applicant Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A first step of forming a first superconductor layer having a desired pattern on a substrate, and a step of forming a desired surface of the first superconductor layer after the first step. A second step of cleaning the surface cleaned by sputtering using an inert gas, and after the second step, high-frequency oxidation treatment on the cleaned surface of the i% 1 superconductor layer. a third step of forming an oxide film as an insulating film to serve as a tunnel barrier; and a fourth step of forming a second superconductor layer extending over the insulating film after the sixth step. The method for manufacturing a tunnel-type Josephson junction is characterized in that after the second step and before the sixth step, the first superconductor layer is heat-treated in an atmosphere containing oxygen. A method for manufacturing tunnel-type Josephson junctions. 2. A first step of forming a first superconductor layer having a pattern of I"lr on the substrate, and after the first step, the above i
'By sputtering the surface of the hair of the AI superconductor layer using an inert gas? On the unified surface/# r
A second step of forming into a reed, and after the second step, a high-frequency oxidation treatment is performed to form an acid ion film as a tunnel barrier on the cleaned surface of the first superconductor layer. In the method for manufacturing a tunnel Josephson junction, the method includes a sixth step, and a fourth step of forming a second superconductor layer extending on the insulating film after the sixth step. Before the sixth stingray scene above, the first superconductor layer is coated with X.
A method for manufacturing a tunnel-type Josephson junction characterized by performing heat treatment in an atmosphere containing oxygen. 3. A first step of forming a first superconductor layer having a required amount of vacuum on the substrate, and after the first step, inactivating the required surface of the first superconductor layer. A second step of cleaning the cloth cleaned by sputtering using waste, and after the second step, high-frequency oxidation treatment on the cleaned surface of the first superconductor layer. a third step of forming a dosing film as an insulating film to serve as a tunnel barrier;
After the third step, a fourth step of forming a second superconductor layer extending on the insulating film. A method for producing a tunnel-type Josephson junction, characterized in that, before step 3, heat treatment is performed on the first superconductor layer in an atmosphere containing oxygen, and then heat treatment is performed in an atmosphere containing oxygen.